No matter how heavily armored a ship is, there are always those tiny weak spots that is necessary for its function, like sensors for turrets and what not. Obviously trying to hit those spots with a starfighter requires some improbable pin point accuracy, and possable help of the Force. Not so for Buzz droids. Once its attached to the hull its virtually impossable to hit at by defence turrets (yes its a serious case of trench run syndrome) All a buzz droid need to do is to get clsoe to one of those devices and make a few correct slices...

No, I never suggest drilling a big hole on blast door or a turret itself. More like jamming the turning mechanism or cracking that targeting sensor or dislocating that forcefield gate.

Obviously the job would be easier if buzzdroids have a built in thermo datonator.